In electrical impedance tomography for regional measurement of pressure-volume ratios, a number of electrodes are placed around the thorax, an alternating current, for example, in the range of 1 kHz to 1 MHz at an amplitude in the range of 1 μA to 10 mA being applied to respective adjacent electrodes. The other respective electrodes are used with the alternating current applied in order to carry out an impedance measurement relative to a defined reference potential. As soon as all the electrodes have been used in turn as current-conducting electrodes, a cycle is completed for data acquisition. In order to eliminate statistical interferences, a plurality of data acquisition cycles are generally averaged in order to obtain a corresponding image. The largest impedance changes in the region of the thorax are produced by the inhalation and exhalation of air. It can be observed in the process that the impedance change measured by the electrodes is a measure for the volume change in the lung. A two-dimensional or even three-dimensional image of the impedance changes can be constructed on the basis of a computer-assisted evaluation of the signals at the electrodes.
The artificial respiration of the diseased lung in which oedemas have formed is a particular problem as it cannot be precisely monitored whether the lung is already closed or collapsed in certain parts. It has been found in this instance that the mortality rate can be substantially lowered if a specific pressure, which just allows all the alveoli to be kept open is artificially maintained in the lung.
For this purpose, WO 00/33733 describes how the alveolar opening and the alveolar closing of the lung can be determined as a function of the respiratory pressure from the measured impedance changes by electrical impedance tomography. However, in this vital application, relatively large measuring errors have to be ruled out as far as possible.
Important measuring errors in electrical impedance tomography are based on the changing impedances of the feed lines to the electrodes and the transition resistances between the skin of the patient and the electrodes. As these interfering impedances are located in series to the impedance to be measured, the interfering impedances directly enter the measurement as errors.
U.S. Pat. No. 5,544,662 describes various measures in terms of circuitry for an evaluation apparatus to reduce the above-mentioned measuring errors. The respective electrodes, however, continue to be connected via feed lines which lead from the electrodes to an evaluation apparatus set up next to the patient, so measuring errors continue to occur owing to impedance changes at the feed lines.
J. D. Bronzino, The Biomedical Engineering Handbook, CRC Press, 1995, pages 745 to 757 describes various types of so-called biopotential-electrodes which are designed as passive electrodes to pick up potentials on the body. It is also mentioned inter alia, that operation amplifiers configured as isolation amplifiers can be integrated on the electrode. Further wiring of the electronics integrated in the electrode is not mentioned, however. This is primarily because the electrodes have to be used as disposable articles for reasons of hygiene and therefore an expensive wiring of the electrode is out of the question for reasons of cost.